The present invention relates generally to speech coding techniques and more specifically to a coded speech communication system.
Araseki, Ozawa, Ono and Ochiai, "Multi-Pulse Excited Speech Coder Based on Maximum Cross-correlation Search Algorithm" (GLOBECOM 83, IEEE Global Telecommunication, 23.3, 1983) describes transmission of coded speech signals at rates lower than 16 kb/s using a coded signal that represents the amplitudes and locations of main, or large-amplitude excitation pulses to be used as a speech source at the receive end for recovery of discrete speech samples as well as a coded filter coefficient that represents the vocal tract of the speech. The amplitudes and locations of the large-amplitude excitation pulses are derived by circuitry which is essentially formed by a subtractor and a feedback circuit which is connected between the output of the subtractor and one input thereof. The feedback circuit includes a weighting filter connected to the output of the subtractor, a calculation circuit, an excitation pulse generator and a synthesis filter. A series of discrete speech samples is applied to the other input of the substractor to detect the difference between it and the output of synthesis filter. The calculation circuit determines the amplitude and location of a pulse to be generated in the excitation circuit and repeats this process to generate subsequent pulses until the energy of the difference at the output of the subtractor is reduced to a minimum. However, the quality of recovered speech of this approach is found to deteriorate significantly as the bit rate is reduced below some point. A similar problem occurs when the input speech is a high pitch voice, such as female voice, because it requires a much greater number of excitation pulses to synthesize the quality of the input speech in a given period of time (or frame) than is required for synthesizing the quality of low-pitch speech signals during that period. Therefore, difficulty has been encountered to reduce the number of excitation pulses for low-bit transmission without sacrificing the quality of recovered speech.
Japanese Laid-Open Patent Publication Sho No. 60-51900 published Mar. 23, 1985 describes a speech encoder in which the auto-correlation of spectral components of input speech samples and the cross-correlation between the input speech samples and the spectral components are determined to synthesize large-amplitude excitation pulses. The fine pitch structure of the input speech samples is also determined to synthesize the auxiliary, or small-amplitude components of the original speech. However, the correlation between small-amplitude components is too low to precisely synthesize such components. In addition, transmission begins with an excitation pulse having a larger amplitude and ends with a pulse having a smaller amplitude that is counted a predetermined number from the first. If a certain upper limit is reached before transmitting the last pulse, the number of small-amplitude excitation pulses that have been transmitted is not sufficient to approximate the original speech. Such a situation is likely to occur often in applications in which the bit is low.